The Dynamic Nature of Brønsted Acid Sites in Cu–Zeolites During NOx Selective Catalytic Reduction: Quantification by Gas-Phase Ammonia Titration
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Brønsted acid sites on Cu-exchanged zeolites can be titrated selectively using gaseous ammonia when NH3 saturation steps are followed by protocols that remove Lewis acid-bound and physisorbed NH3, such as purging in flowing wet helium at 433 K. NH3 titrates all H+ sites on small-pore chabazite zeolites (SSZ-13) and leads to the complete disappearance of infrared stretches for Brønsted acidic OH groups after saturation (433 K), in contrast with larger n-propylamine titrants that access only a small fraction (<0.25) of H+ sites on SSZ-13 under conditions sufficient to titrate all H+ sites on medium-pore ZSM-5 zeolites (323 K, 2 h). NH3 titration of the residual H+ sites present in Cu-exchanged SSZ-13 samples (Si/Al = 4.5, Cu/Al = 0–0.20) after oxidative treatments detects two fewer H+ sites per exchanged Cu2+ ion, as expected to maintain framework charge neutrality. NH3 titrants detect only one fewer H+ site (per Cu) after Cu-SSZ-13 samples undergo a reductive treatment in flowing NO and NH3 (473 K), however, indicating that each Cu2+ cation reduces to form a Cu+ and H+ site pair. In the context of low temperature (473 K) selective catalytic reduction (SCR) on high aluminum Cu-SSZ-13, we discuss the different mechanistic roles of residual H+ sites that remain after Cu2+ exchange, whose primary function appears to be NH3 storage, and of proximal H+ sites that are generated in situ upon Cu2+ reduction, whose role is to stabilize reactive NH4 + intermediates involved in the standard SCR oxidation half-cycle. We highlight how gaseous NH3 titrants can selectively count H+ sites on small-pore, Cu-exchanged zeolites and, in doing so, enable probing the dynamic nature of active sites and catalytic surfaces during SCR redox cycles.
KeywordsAmmonia Brønsted acid site Copper-exchanged zeolites n-Propylamine Selective catalytic reduction Titration
We acknowledge the financial support provided by the National Science Foundation GOALI program under award number 1258715-CBET. RG also acknowledges financial support from a Ralph E. Powe Junior Faculty Enhancement Award from the Oak Ridge Associated Universities, and from a Purdue Research Foundation Summer Faculty Grant. Support for JTM was provided under the auspices of the U.S. DOE, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract number DE-AC0-06CH11357. We would like to thank Sachem, Inc. for their donation of the structure-directing agent used to synthesize SSZ-13, Dr. Yury Zvinevich for assistance constructing a custom-built acid site titration unit, Austin Tackaberry for assistance with SSZ-13 sample preparation, and Arthur Shih and Jonatan Albarracin-Caballero for assistance with some of the NH3 TPD experiments. Finally, we would like to thank Professor Mark E. Davis for continuing to lead by example and inspire his current and former colleagues to pursue creative research problems in catalysis.
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